The present invention is directed to xe2x80x9cactive implantable medical devicesxe2x80x9d as such devices are defined by the Jun. 20, 1990 directive 90/385/CEE of the Council of the European Communities, more precisely to pacemaker, defibrillator and/or cardiovertor devices which are able to deliver to the heart pulses of low energy for the treatment of the cardiac rate disorders, and even more particularly, to the so-called xe2x80x9cmultisitexe2x80x9d prostheses in which electrodes are placed in a plurality of distinct respective sites, comprising at least one ventricular site and one atrial site.
Multisite prosthesis are known and include the following general types: xe2x80x9cdouble chamberxe2x80x9d (right atrial stimulation and right ventricular stimulation), xe2x80x9ctriple chamberxe2x80x9d (right atrial stimulation and double ventricular stimulation), and xe2x80x9cquadruple chamberxe2x80x9d (double atrial stimulation and double ventricular stimulation).
The control of stimulation implies that there is a continuous adjustment of various parameters such as the stimulation frequency, the atrio-ventricular delay (AVD), or the inter-ventricular delay in the case of a bi-ventricular stimulation. These various parameters are adjusted according to signals delivered by sensors, for example, a measure of the well known minute-ventilation (MV), which is a cardiac parameter representative of the instantaneous metabolic needs of the patient.
Another parameter which is interesting to know is the cardiac flow. It can be particularly interesting with multisite-type pacemakers to obtain an indication of the cardiac flow, and thus of the ejection fraction, which is a well known hemodynamic reference parameter, in order to optimize stimulation of the various sites.
International patent application WO-A-99/34863 (assigned to Pacesetter AB) describes a lead allowing one to obtain an indication of this parameter by a measurement of the intracardiac pressure, using a piezoelectric sensor incorporated at the end of a lead. The pressure is integrated between the moment of opening and the moment of closing of the valve, which gives an indication of the work provided by the cardiac muscle. This information is then used, inter alia, to control the stimulation frequency and the atrio-ventricular delay. This manner of proceeding, however, if it is to be effective, requires a specific lead incorporating a piezoelectric sensor, as well as a particular electronic circuit to condition the signals produced by the piezoelectric sensor, to convert them and transmit them to the microprocessor of the pacemaker.
Another parameter which can be measured and correlated with the cardiac flow is the trans-valvular impedance, generally measured at the right heart. In this respect, Chirife U.S. Pat. No. 5,154,171 (Chirife) describes a manner of taking a dynamic measurement of bio-impedance, allowing one to evaluate the diastolic and systolic volumes, and to obtain therefrom an indication of the cardiac flow, and thus of the ejection fraction. The signal obtained is then used to control the heart rate so as to adjust the heart rate up or down in the direction that will obtain a maximization of the cardiac flow.
The Chirife patent proposes to make the bio-impedance measurement by injecting a current pulse between two points, and collecting (detecting) a differential potential between the same two points. In practice, however, it has been discovered that this configuration of injection/collection appears sensitive to the movement of the leads, and therefore does not allow a reliable and precise measurement of the trans-valvular impedance.
It is, therefore, an object of the present invention to overcome the aforementioned disadvantages by proposing a new measurement configuration for the trans-valvular impedance with a particular choice for the injection and collection sites.
Another object of the present invention is to use the parameter thus measured to control the inter-ventricular delay, in the case of a bi-ventricular stimulation (the trans-valvular impedance also being able of course to control the stimulation frequency and/or the atrio-ventricular delay).
The present invention is thus directed to an active implantable medical device, in particular a pacemaker, defibrillator and/or cardiovertor and/or a multisite device, in which electrodes are placed in a plurality of respective distinct sites comprising at least one ventricular site and one atrial site on the same side of the heart, these electrodes being connected to at least one circuit for the collection (detection) of cardiac signals, the collection circuit being able to detect depolarisation potentials including a differential potential between two electrodes, as well as to a stimulation circuit to apply stimulation pulses to at least certain ones of the aforesaid sites. This device further comprises means for evaluating the cardiac flow based upon a measurement of the trans-valvular bio-impedance, which means operates by injection of a current between an atrial site and a ventricular site, and the collection of a differential potential between an atrial site and a ventricular site.
According to the invention, a tripolar electrode configuration is used to obtain the trans-valvular bio-impedance, with one site common to the injection and the collection, one site dedicated to the injection, and one site dedicated to the collection. The common site is located in one of the cavities, and the two dedicated sites are located in the other cavity.
In a preferred embodiment, the device also comprises means for varying the inter-ventricular delay of the application of the respective stimulation pulses on the right and left ventricles (in the case of a multisite device), and/or means for varying the frequency of application of the stimulation pulses, and/or means for varying the atrio-ventricular delay in the application of the stimulation pulses. These various means, when employed independently or collectively, operate in response to the measured trans-valvular impedance to vary the control parameter in a direction that results in an improvement of the cardiac flow.
In one embodiment of the invention, the common site is an atrial site and the two dedicated sites are ventricular sites.
In another embodiment, the common site is a ventricular site and the two dedicated sites are atrial sites.
In yet another embodiment, the common site is a proximal electrode of a lead, and in yet another embodiment, the two dedicated sites are a proximal electrode and a distal electrode of the same lead, which may be an atrial site or a ventricular site.
In a preferred embodiment of the present invention, the site common to the injection and the collection is a proximal electrode of a right atrial lead, the site dedicated for the injection is a distal electrode of a right ventricular lead, and the site dedicated for the collection is a proximal electrode of the same right ventricular lead.